This invention relates to electronic image scanning and processing and, more particularly, to the calibration of photodetector output to compensate for intensity and color temperature variation in subject illumination by a low-cost source of illumination.
In a commonly assigned, co-pending U.S. patent application entitled "COLOR BALANCED IMAGE DETECTOR SYSTEM", Ser. No. 328,314, filed Mar. 13, 1989, there is disclosed a color balanced imaging system in which plural (e.g. 3) sets of photodetectors are exposed to a light image reflected from a subject, which may be represented by a color photograph, for example, in a manner to achieve a high resolution, accurate color rendition of the image in digital form. The photodetector sets are exposed by scanning the subject image with white light so that light reflected from a given picture element or "pixel" in the subject image will impinge each individual photodetector located in the scanning path. To this end, the photodetectors in each set are arranged in adjacent rows transverse to the scanning path and in columns parallel to that path. Where the subject reflects only light in the visible spectrum, each set of the photodetectors is exposed through a filter which could represent one of the primary colors, that is, red (R), green (G), or blue (B). Alternatively, the filters could represent the complimentary colors.
Color balance in the system of the aforementioned co-pending application is achieved by a combination of selecting the number of photodetector rows in each set to correspond with the sensitivity of the photodetectors to the light in the R, G and B spectral regions and by accumulating the charge developed by photodetectors in each column of each set so that the output signal for each subject image pixel is the composite output of the photodetector rows. For example, the photodetectors are most sensitive to red light and least sensitive to blue light. Thus, the photodetector set exposed through the red filter will have as few as six active rows of photodetectors where as the blue set will have as many as sixteen photodetector rows. As light reflected from each image pixel scans or traverses the photodetectors in a column common to all three sets, the R, G or B spectral components in the pixel develop a charge in each photodetector underlying a filter which passes a spectral component in the pixel. The photodetector charges are accumulated in each photodetector set in a manner such that each successive photodetector in a column receives the charge of a photodetector immediately preceding it in the context of scanning direction and also adds its own charge to the accumulating charge. As a result of the accumulation of charges, each image pixel is represented by a well developed photodetector signal capable of being processed electronically to develop a digital emulation of the subject image. Moreover, an exceedingly high measure of color balance is achieved simply by variation in the number of photodetectors exposed to the respective R, G and B spectral components of each subject image pixel.
While the afore-mentioned scanning system represents a major advance in the attainment of data accurate with respect to color balance, the overall quality of data obtained by image scanning is dependent also on uniform intensity and color temperature of the source of the illumination reflected from the subject image to the photodetector sets. If a low-cost tungsten halogen bulb powered by line voltage, for example, is used as the source of image scanning illumination, the intensity of light reflected from the subject image to the photodetectors will vary with normally incurred line voltage variations including the sixty cycle ripple associated with line voltage. Also, the color temperature of such a light source is unstable particularly during the period immediately after the lamp is turned on. Quite obviously, the signals generated by the photodetectors exposed to such a light source will vary with the voltage variation induced intensity of the light, and with color temperature variation where photodetector signal strength is predicated on response to a limited spectrum of the reflected light.
The problems associated with intensity and color temperature variation in low-cost light sources for image scanning applications have been addressed in the prior art. For example, U.S. Pat. No. 4,174,528 and the prior art cited therein as background art teach the use of a white reference image or patch in the light path between the light source and a photodetector array for the purpose of calibrating the output of the array to compensate for variations in exposure light intensity and color temperature. In many of the prior art disclosures, the white reference patch is exposed to the photodetectors only at the beginning of an image scan. It is obvious that this approach to photodetector calibration is effective only in a very general way and would not account for cyclical variations in light intensity variations which occur during the scan of a single subject image. The disclosure of U.S. Pat. No. 4,174,528, on the other hand, approaches a scanner calibration procedure by which photodetector calibration is accomplished on a line-by-line basis. Specifically, a linear detector array provided for reading the document or subject image includes a detector in the path of light reflected from reference indicia and associated with an exposure meter circuit. When light reflected from a given line of image pixels is read by the photodetector array, the quantity of light read by the exposure meter associated detector is integrated and stored in a manner to control the exposure of the image sensing photodetectors in the next line of pixels. This system operates in the manner of an electronic shutter by which the time during which the detectors are exposed to light reflected from the subject image is controlled by the exposure meter circuit.
From the standpoint of adapting the exposure control arrangement of the afore-mentioned U.S. Pat. No. 4,174,528 to the system described with reference to the afore-mentioned co-pending patent application in which the charges developed by individual rows of photodetector cells are accumulated to achieve color balance, several problems arise. For example, where the photodetector cell charges are accumulated in successive rows, it is essential that the rate of photodetector charge transfer is identical to the rate of image pixel scan. Although it might be possible to vary the rate of scan to accommodate a variable exposure time interval for each row of photodetectors, the costs of doing so are likely to be greater than the costs of a non-variable light source. Also the resolution requirements for reproducing a subject image represented by a color photograph, for example, require a photodetector cell density of a magnitude making it extremely difficult to accommodate circuity associated with electronic shutter operation. Finally, the attainment of digital data capable of reproducing a high resolution subject image, such as a color photograph, requires that photodetector cell calibration be accomplished on a real time basis, that is, so that the photodetector cells in each row scanned by the subject image are calibrated on the basis of light to which they are actually exposed.
In light of the foregoing, it will be seen that there is a need for an effective photodetector calibration system for the color balanced scanning technique of the type in which photodetector output is the result of the transfer and accumulation of successive cell charges.